A while ago I wrote about how bacteria make their way into clouds, where they act as seeds around which raindrops condense. Now, a team of scientists in France has shown that the microbes floating around in the clouds do more than just make it rain to bring them back down to the surface; they also carry out chemistry while they’re up there.

Though they may appear serenely inert from our distant vantage, clouds are active, constantly changing objects; rich with chemicals, exposed to high levels of UV radiation and made up of a mixture of liquid water, ice and water vapour, they’re ideal and dynamic chemistry labs. The possibility that bacteria and other micro-organisms might be involved in the chemical reactions taking place in the clouds isn’t actually a new idea. Although clouds are cold, dry and acidic, bacteria have been known to remain active even under such inhospitable conditions. Over the past five years the team of researchers collected water samples from clouds and showed that the microbes in them could carry out the kind of chemistry that would influence the composition of the clouds. An important factor was left out of those studies, though; the chemical reactions and high UV levels in clouds produce hydrogen peroxide (often used as bleach) and other reactive, toxic chemicals which were missing from the microbial cultures grown in the lab. The team addressed this in their latest study, extending their earlier work to find out if and how cloud-borne microbes would cope with such conditions.

To answer that question, the team collected cloud water from Puy de Dôme in central France and grew the samples in custom-built growth chambers designed to mimic the conditions found in clouds. To tease apart the effect of the microbes from all the other chemistry going on, they filtered half of the samples to remove any micro-organisms in them. The filtered and unfiltered samples were then grown with and without UV light; by comparing how the chemical composition changed over time under these different conditions, the researchers could separate the reactions that were driven by UV light or microbial activity from the background chemistry going on all the time.

It turned out that the micro-organisms were able to drive a lot of important chemical reactions regardless of whether or not they were exposed to UV light. Within two or three days, they broke down many of the carboxylic acids that contain a lot of the carbon trapped in clouds. Another important component of clouds, formaldehyde, was broken down by the microbes but assembled by light-driven chemical reactions; when both UV light and microbes were present, the synthesis and breakdown balanced out and the amount of formaldehyde didn’t change.

The researchers also tracked the energy metabolism of the micro-organisms to measure how much stress the UV light and hydrogen peroxide were causing. The energetic state of the microbes improved over time and, importantly, was the same in the presence or absence of UV light. This means that the microbes weren’t stressed by the addition of UV light and the increased reactive chemicals it produced. The researchers also measured the amount of hydrogen peroxide in the different treatments and found that the microbes were protecting themselves by breaking it down.

Like the clouds they’re floating in, these microbes are far from inert. This study shows that they’re able to cope with the harsh environment in the clouds and stay metabolically and chemically active. Unaffected by the high UV levels and reactive chemicals around them, they keep carrying out chemical reactions in the clouds day and night. These reactions, which affect the composition of the clouds, could have a major impact on atmospheric chemistry and will have to be included in our atmospheric models. Aside from amazement at the bacterial chemistry labs in the sky, to me this serves as another reminder that, despite humanity’s sense of aggrandisement, bacteria are the true rulers the planet.

Thanks, I’m glad you enjoyed the post! Learning about all this has definitely changed the way I see clouds, which just makes me wonder what other great stuff I’m missing…it really is a wide and wonderful world out there!🙂